From first twenty natural numbers, 2 numbers are selected at random, the probability that the selected numbers are such that their sum is even is

n(S)= 20C2 Sum is even if, both are even or both are odd⇒n(E) = 10C2+10C2                ↓               ↓          both even    both Odd P(E) = 10C2+10C220C2  ∴ The probability that the selected numbers are such that their sum is even is  10C2+ 10C220C2